I have never build BMX wheels, but used the TM-1 a lot. There is no need to really center the meter on the spoke. The meter functions on the principle that if you apply a force perpendicular to a beam at the center of two support points, the deflection is dependent on the applied force F and on the force pulling in the axis of the beam, on the distance L between the support points (about L^3 ; L= 105 mm for the TM-1), on the beam diameter (about D^-4) and on the elastic modulus of the spoke material. The length of beam outside the support points is less important (except less than 5 mm).
You can check this by taking a common 28" bicycle wheel and take readings while moving the TM-1 from close to the nipple to close to the spoke cross.